The report of the detention of Xu Zhiyong, a civil rights lawyer, was a reminder that, for all the attention given to Mr Ai’s release, many more activists are still being held.

The scores of formal arrests, unofficial detentions and unexplained disappearances in recent months mark China’s most intense period of repression since the crackdown following the Tiananmen protests in 1989, campaigners say ….

“There is no doubt that, after the Arab spring, the authorities launched a comprehensive effort to redefine the limits of permissible expression,” said Nicholas Bequelin, of Human Rights Watch in Hong Kong. “The government came to the conclusion that the only way to rein in criticism was to physically take the critics off the grid,” he said ….

Mr Xu, the lawyer, went missing on Wednesday, according to Human Rights Watch. A news official in the national public security bureau said she was not aware of any arrest. Mr Xu’s phone was turned off. Previously jailed in 2009, Mr Xu had been helping people trying to run as independents in local elections.

Xu’s law firm was one of the few in China willing to represent the parents of the nearly 300,000 children sickened and the six who died last year as a result of dangerous milk additives.

Since its founding in 2003, the firm, also known as Gongmeng, has not shied away from sensitive topics. It challenged China’s secret detention centers, the so-called black jails, after a 27-year-old graphic designer who was arrested for failing to carry his identification card died in custody. Xu represented an editor of the hard-hitting newspaper Southern Metropolis Daily who was arrested in 2004 on what were widely seen as politically motivated bribery charges.

This summer, Xu’s firm joined the chorus of voices opposing a requirement that all computers sold in China come preinstalled with software that would filter out pornographic or controversial content.

But Xu is by no means a dissident, preferring to work within a system he has hoped to improve, not overthrow.